This is indeed a very special episode, an episode in which Jack appears for only a couple of minutes and has absolutely no lines, and the star of the episode is a former killer, a robot now retired who was capable of emotions.
The story is told through the eyes of this robot, X-9, a former killer, one of the first robots created for Aku by various mad scientists. One of this scientists, "the funny one", implants an experimental emotion chip into X-9. With time, X-9 develops feelings, starts to think before he acts (unlike the other robots who because of this are easily destroyed in battles), until he feels he has done to much evil and, after he meets a dog he calls Lulu, he decides to retire. His life changes, and now he is for the first time ever happy.
However Aku learns that this robot is capable of emotions so he kidnaps Lulu, in order to blackmail X-9 into coming back to work for him once more, his first mission being the destruction of Samurai Jack. X-9 reluctantly accepts, and tracks Jack to an abandoned robot construction factory. There, after a chase, Jack manages to defeat the robot, not knowing his truly impressive back story. With his last words, X-9 says "Please take care of Lulu. Sweet thing."
The episode ends with raindrops falling on a picture of Lulu. An exciting and very special episode of Samurai Jack, which mustn't be missed for anything in the world.